Most popular cactus sold?

[Mrs.Green]

One Windowsill; Aloe vera is a good guess but I would belive that the ‘Cowboy cactus’ ( Euphorbia trigona) as I often sees it named as, is high on the list?

One to burn the skin and the other to heal it?

Exactly!

[promethean_spark]

Definitely the edible prickly pear is most common hereabouts, people plant it even in parks and ditches for later harvest - basically guerilla gardening. They're also common along fence lines on ag land for the same reason.

I'd say echinopsis/trichocereus are the most common ornamentals - unlike typical Mams/gymnos ect in 3" pots, most folk can keep the 'easter Lily cactus's alive. I have a lot of them as I can leave them out in the rain all winter, so they don't cost gh space. I got my first couple as a teen and still have them. Those are also by far the most common I see in larger pots for sale.

[DaveW]

Yes, conditions dictate popularity. The Brits tend to grow smaller species that take up less room in a heated greenhouse rather than taller Cerei or big Opuntia's which more of us would do if we could grow them outside all year.

[Aeonium2003]

Opuntia are definitely some of the most common cacti (here at least). They are also partially invasive.

However, I don't see them being sold too often.